The production of chemical energy from solar energy, or the use of inexhaustible and clean hydrogen energy is one of the dreams which the human beings have. The practical application of this energy can solve the energy problem faced by the 21st century, the warming of the earth by carbon dioxide as brought about by fossil energy and environmental pollution as by acid rain.
The Honda-Fujishima effect announced by A. Fujishima, et al, Nature, 238, 37 (1972) was the first attempt to show that the energy of light can be used to decompose water into oxygen and hydrogen. When the oil crisis was thereafter questioned all over the world, a lot of studies were actively made for converting the energy of light to chemical energy in accordance with that principle. The conversion efficiency for the energy of light in the visible range is, however, still to be improved. The result of the active studies made from 1980 to 1990 is that they have shown that the conversion efficiency is dictated by the recombination of electrons and holes formed by photoexcitation when it occurs before they reach the reaction sites for the decomposition of water. In accordance with this conclusion, an attempt has been made to use an intercalation compound to separate the reaction sites (S. Ikeda, et al, J. Mater. Res., 13, 852 (1998)). No satisfactory conversion efficiency in the visible range has, however, been achieved as yet, though the conversion efficiency has gradually been improved. This is due to the fact that no perfect separation of the reaction sites or of electrons and holes has been achieved.
Besides the studies mentioned above, there have also been made studies of reaction systems for generating hydrogen by utilizing the absorption of light by ions in a solution. J. Jortner, et al, J. Phys. Chem., 68, 247 (1964) showed the generation of hydrogen at a high quantum efficiency in an acid solution containing an iodine ion, and K. Hara, et al, J. Photochem. Photobiolo., A128, 27 (1999) in an alkaline solution containing a sulfur ion. All of these reactions are, however, made possible by ultraviolet light of high energy having a wavelength of 250 nm or less.
As to the application of photocatalytic technology, its practical application has begun in antibacterial tiles, antibacterial and deodorant filters for air cleaners, etc. owing to its property of promoting various chemical reactions, such as the degradation of environmental pollutants, sources of offensive odors and bacteria. Moreover, it is possible to obtain a useful chemical substance by reacting a harmful substance with a photocatalyst. For example, application to a desulfurization process for crude oil will be possible.
The desulfurization process for crude oil which is generally employed at present hydrogenates heavy naphtha during the distillation of crude oil and recovers all the sulfur components of crude oil as hydrogen sulfide. It recovers hydrogen sulfide by oxidizing sulfur through a process called the Claus process. The Claus process is a process which oxidizes one-third of hydrogen sulfide into sulfur dioxide and reacts it with the remaining hydrogen sulfide to form elemental sulfur.
This process requires enormous energy, since it not only carries out the catalytic reaction of sulfur dioxide and hydrogen sulfide, but also repeats heating and condensation. Other problems thereof include the expensive control of sulfur dioxide.
If it is possible to put into practice a method in which a photocatalyst is added to alkaline water containing hydrogen sulfide dissolved therein, and is irradiated with ultraviolet rays, so that free electrons and holes generated by the photocatalyst absorbing the energy of ultraviolet light may oxidize and reduce the alkaline water containing hydrogen sulfide to produce hydrogen and sulfur, or a method in which hydrogen sulfide is decomposed by a photocatalyst into hydrogen and sulfur, it will be possible to decompose hydrogen sulfide as an harmful substance and produce hydrogen and sulfur as useful substances with a smaller amount of energy. In other words, it will be possible to contribute to solving an environmental problem and produce useful substances inexpensively.
The photocatalysts so far available have, however, had problems to be solved, as will now be stated. Firstly, they are low in catalytic activity. Secondly, they are toxic. They are low in catalytic activity, since it is highly probable that the free electrons and holes which are formed upon application of light to a photocatalyst may undergo recombination, and it is also highly probable that the chemical substances separated by an oxidation-reduction reaction may recombine into the original compound.
Thirdly, the catalysts have only a short life. They have a problem of being dissolved by light. The free electrons and holes which are formed upon application of light to a photocatalyst produce a strong oxidation-reduction reaction by which not only the chemical substances as intended, but also the catalyst itself is oxidized and reduced, is dissolved and loses its catalytic action.
Under these circumstances, JP-A-2001-190964 has disclosed a photocatalyst having a high catalytic activity, having no toxicity and having a long life, and overcome the three problems as stated above.
JP-A-2001-190964, however, discloses only a photo-catalyst composed of ZnS. As the band gap of ZnS is in the ultraviolet range, it has been impossible to utilize visible light, such as solar light, an inexhaustible and clean source of energy, directly for a photocatalytic reaction.
It is, therefore, an object of this invention to overcome the drawbacks of the prior art as stated above and provide a photocatalyst which is high in catalytic activity, is nontoxic, has a long life, allows visible light to be used directly for a photocatalytic reaction and is especially useful for hydrogen generation, and-a process for producing the same.